From electron-microscopical observations, a decreased metabolic activity in 3-day-old Candida albicans chlamydospores was suggested, and progressive deterioration in chlamydospores aged 2-8 months was shown. Oxygen utilization by chlamydospore-pseudomycelium (CSP) preparations was less than that by yeast, while 3-day-old CSP preparations used significantly less O2 than 24-h CSP preparations. Amino acid incorporation was greater in yeast than in CSP preparations. Leucine incorporation by 20-h yeasts was twice that of 5-day yeasts and 5 times that of 20-h and 5-day CSP. Amino acid decarboxylation was similar in yeasts and CSP and was determined by end-product analyses to be via amino acid oxidase. Light microscopy autoradiography of [14C]leucine incorporation demonstrated that the metabolic activity in CSP preparations was due to the young growing tips of the pseudomycelium and not to mature chlamydospores. Yeasts did not take up trypan blue could be stained if first autoclaved or treated with 10% acid or 10% base. Young chlamydospores grown in the presence of trypan blue developed unstained and became permeable to the dye at 2 1/2 days. These data suggest that chlamydospores of C. albicans do not function in the classical role attributed to spores; i.e., mature chlamydospores cannot germinate, but rather age, deteriorate, and die.